1. Field of the Invention
The present invention relates to the field of electronic technology, and more particularly to a power factor correction (PFC) control circuit and a power adapter of an electronic device such as a liquid crystal display (LCD), a television (TV) or a light emitting diode (LED) lamp.
2. Description of the Related Art
To provide an appropriate and stable power to an electronic device, the electronic device is usually equipped with a power adapter. In general, most conventional power adaptors adopt a pulse width modulation (PWM) for controlling the voltage or current value of an output power supply. Power factor (PF) refers to the relation between effective power and total power consumption (or apparent power), or the ratio of the effective power to the total power consumption (or apparent power), which is used for utilizing a constant power effectively, and thus the power factor is an important indicator of the properties of a constant power adapter. To enhance the PF property, the utilization of electric power, a power adapter generally comes with a PFC control circuit.
With reference to FIG. 1 for a conventional PFC control circuit, and the conventional PFC control circuit comprises a rectifier 10, a coupling transformer 11, a current limiting switch 12, a first resistor 13, a second resistor 14, a controller 15 and a current limiter 16, wherein the second resistor 14, the controller 15 and the current limiter 16 are operated at a discontinuous conduction mode (DCM) to output a stable current and prevent an over-current. The coupling transformer 11 comprises a primary coil, a secondary coil and an auxiliary coil NA, and the controller 15 comprise a first comparator 151, a second comparator 152, a logic gate 153, a trigger 154 and an optical coupler (OC) 155, and the current limiter 16 comprises a first multiplier 161 and a second multiplier 162. The current limiting switch 12 is a triode having a collector coupled to the primary coil, and a base coupled to an output terminal of the trigger 154, and an end of the first resistor 13 is grounded and the other end of the first resistor 14 is coupled to an emitter of the current limiting switch 12. The first multiplier 161 and the second multiplier 162 of the current limiter 16 are coupled to output terminals of the trigger respectively. The rectifier 10 converts the utility power (AC) into a primary current Ip (DC), and then input voltage are formed at both ends of the input voltage of the primary coil of the coupling transformer 11, and the secondary coil generates an induced current Is, and the auxiliary coil generates a detect current Ia, and the first resistor 13 senses the primary current Ip to output a sensing signal to the controller 15. The auxiliary coil is serially coupled to the second resistor 14, so that the second resistor 14 can sense the primary current Ip to output a detect signal to the controller 15. After the first multiplier 161 and the second multiplier 162 compute the received PWM signal and reference signal, a compare signal increasing with time is outputted to the controller 15, and the controller 15 processes the sensing signal, the detect signal, the compare signal and the coupling signal outputted from the optical coupler 155 to generate and transmit a setting signal or a clear signal to the current limiting switch 12, and the current limiting switch 12 switches its status according to the received setting signal or clear signal to control the secondary coil, so as to generate a stable induced current Is and overcome the over-current issue caused by the circuit latency effect. Although the aforementioned PFC control circuit provides the over-current protection effect, yet the circuit employs the optical coupler which incurs a larger volume and a higher cost, and the multiplication circuit also causes an input voltage harmonic in the output current of the entire circuit, and results in a greater total harmonic distortion (THD) and a smaller PF property, and thus wasting energy.